Atmospheric response to Arctic sea-ice loss dependent on ENSO and QBO

The Polar Amplification Model Intercomparison Project (PAMIP) simulations have revealed large differences in the atmospheric responses to Arctic sea-ice loss across models. The atmospheric response to sea-ice loss may be sensitive to the background state, which differs between models. Work is presented here that expands the PAMIP experiments by testing the sensitivity of the response to sea-ice loss in HadGEM3 to the initial atmospheric conditions (namely, the Quasi-Biennial Oscillation, QBO) and the background SST state (specifically, the El Niño Southern Oscillation, ENSO). The stratospheric polar vortex in the northern hemisphere weakens in response to Arctic sea-ice loss during QBO easterly but strengthens in response to Arctic sea-ice loss during QBO westerly. The polar vortex state is not found to be as strongly modulated by ENSO as the QBO. During El Niño with QBO westerly however, there is an observed change in seasonality of the tropospheric response to sea-ice loss compared to the neutral ENSO state. A weakening and equatorward shift of the mid-latitude jet stream in response to sea-ice loss is most pronounced in autumn under the El Niño state, but occurs most strongly in winter under the neutral ENSO state.